Sunday, April 26, 2009

During the afternoon of the last day of the European Geophysical Union meeting in Vienna, I had two very interesting conversations, one with a member of the IRIS-PASSCAL polar team, and the other with a friendly glaciologist I knew from Concordia.

The polar team at IRIS-PASSCAL have done all the development and part of the deployment of the American experiments in Antarctica for the International Polar Year. Their station designs are first rate, and their success rate for last season makes me envious (23 out of 24 stations deployed worked perfectly, sending state of health data over satellite links). We exchanged design details, and did some troubleshooting for some of the instruments at Concordia. One very interesting, motivating and helpful conversation!

Just afterwards, I bumped into a glaciologist from LGGE in Grenoble, who I met for the first time at Concordia, two years ago. We got talking about the ice structure around Dome C (quite a lot is known thanks to the EPICA ice core), and how it may influence the seismic data we record up there. He gave me a good overview of how compaction works, and suggested some people who might know more about the mechanical properties of the various ice layers, their density and possibly their wavespeeds. This might help us figure out how to correct for the ice-signal on or recorded seismograms.

Just an example of how putting a bunch of scientists and technical people in a conference center together can turn out to be hugely profitable, often in unpredictable ways.

Monday, April 20, 2009

One of the reasons for installing seismic stations in Antarctica is to improve our knowledge of the Earth's inner core. It would seem - from the data currently available - that seismic waves that propagate through the inner core parallel to the Earth's rotation axis (polar paths) are faster than those that propagate perpendicular to the Earth's rotation axis (equatorial paths). This speed difference implies the inner core is anisotropic, with a N-S fast axis.

This conclusion is based on a large number of equatorial paths, and only a small number of polar paths, most of which come from earthquakes in one particular region (South Sandwich islands) recorded in Alaska. Data from new stations in Antarctica are expected to increase the number of polar paths available, and improve our understanding of the inner core.

At EGU this morning, I heard one of the first talks on the inner core that actually uses data from newly deployed Antarctic stations:

The data are from the SSCUA stations, deployed near Mawson station. The PKP(bc-df) measurements made on these data are all between 0 and 2 seconds, and imply that if there is N-S oriented anisotropy in the inner core, this anisotropy must be weak (and specifically, much weaker than implied by the South Sandwich data). Indeed, the weak anisotropy hypothesis seems to be consistent with all data except those from South Sandwich.

Should we therefore dismiss anisotropy in the inner core? Possibly. But before doing so we need more data and measurements from other Antarctic stations (the CASE-IPY and Concordia stations will contribute some of these data), and we need to understand why the PKP(bc-df) measurements from South Sandwich events are so large.